Referring to FIG. 1, most known pressure sensing MEMS sensing dies are composed of dual stack dies manufactured by anodic bonding two wafers:                a silicon wafer 11: a silicon wafer bulk-machined to have at least one thin diaphragm 15 supported by a rigid outer frame 17. Strain gauges, or piezoresistors, 16 are placed on or in the diaphragm 15 in order to sense the stresses produced in the diaphragm 15 by applied pressure;        a Pyrex® wafer 12: a relatively thick Pyrex® wafer acts as a pedestal to provide support to the silicon wafer 11.        
In order to produce a pressure sensor assembly, the Pyrex® pedestal 12 of the dual stack die is normally mounted with an adhesive 18 on a base 13, the base being typically a metal header, or a ceramic or plastic substrate.
Since the thermal coefficient of expansion of Pyrex® is significantly lower than metal or ceramic, the thermal mismatch induces significant compression stresses in the Pyrex® pedestal 12 when the sensor returns to ambient temperature. The locked-in compression stresses, referred to as die-attach stresses, deform the Pyrex® and the outer frame 17 which, in turn, forces the sensing diaphragm 15 to deflect. The deflection is sensed by the diaphragm piezoresistors 15, which generate a die-attach residual null offset output proportional to the locked-in die-attach stresses.
Now, since in most applications the pressure sensors are required to work in a wide range of temperatures, the adhesive 18 used to attach the Pyrex® pedestal 12 onto the base 13 is made of an elastic material, such as RTV.
However, since the RTV is not perfectly elastic and suffers from temperature hysteresis, the locked-in residual null offset does to return to its original value after a temperature cycle.
This appears as short term instability and drift.
In addition, ongoing bond relaxation and RTV aging produce long term effects on the die-attach stresses, which result in long term drifts off sensor offset or null output voltage.
It is to be noticed that these drifts problems are exacerbated in low pressure range sensors that normally use relatively thin diaphragms.
Earlier attempts to isolate the die-attach drift problem involved etching slots or channels in the silicon die in the sensing diaphragm (US 2001/0001550 A1) or adding a plurality of relief channels etched in an upper and a lower surface of an intermediate layer (U.S. Pat. No. 6,822,318 B2).
Since the geometry of channels is dictated by the limited thickness of silicon wafer used in MEMS sensors, this approach requires multiple upper and a lower surface channels is slots with very thin webs, which is problematical and costly to implement.
Furthermore, etching slots or channels in the silicon die around the sensing diaphragm in close proximity to the piezoresistors may create stability problems.